nomad/structs is an internal package and imports many libraries (e.g.
raft, codec) that are not relevant to api clients, and may cause
unnecessary dependency pain (e.g. `github.com/ugorji/go/codec`
version is very old now).

Here, we add a code generator that imports the relevant constants from
`nomad/structs`.

I considered using this approach for other structs, but didn't find a
quick viable way to reduce duplication.  `nomad/structs` use values as
struct fields (e.g. `string`), while `api` uses value pointer (e.g.
`*string`) instead.  Also, sometimes, `api` structs contain deprecated
fields or additional documentation, so simple copy-paste doesn't work.
For these reasons, I opt to keep the status quo.